Crossing International Date Line

The International Date Line runs from the North Pole to the South Pole and distinguishes the change of one calendar day to the next, passing through the middle of the Pacific Ocean, roughly following the 180° line of longitude, and is exactly halfway round the planet from Greenwich, London.

So, for guests sailing West on their World Cruise, it will result in a 'Lost Day', due to location of this imaginary line, and so it follows that for guests sailing East, it results in a day gained.